1. Field of the Invention
The present invention relates to a device for adjusting the dioptry of a viewfinder eyepiece of a single-lens reflex camera.
2. Description of the Prior Art
The viewfinder eyepiece of single-lens reflex cameras is generally standardized to form images at about -1 diopeter (one meter ahead of the eye). It is often difficult for near-sighted, far-sighted or presbyopic persons to observe with their naked eye an image formed one meter in front of the eye, the difficulty varying with the individual. This, in turn, makes it difficult for those persons to properly adjust the focusing of the objective lens of a single-lens reflex camera. The problem of dioptry may be avoided by using spectacles, but this requires holding the eye physically away from the eyepiece, whereby the visual field is eclipsed or is hard to observe. For optical instruments, therefore, there have been proposed dioptry adjusting devices for adjusting an image to be formed at an easy-to-see position in conformity to the power of accommodation or adjusting faculties of the eye of the individuals. One typical method is to move the eyepiece along the optical axis and this method has been employed in the viewfinders for cine cameras, binoculars, microscopes, or the like. As a rule, if an eye piece of focal length f is displaced by .DELTA.d then the amount of diopter variation can be represented by 1000.multidot..DELTA.d/f.sup.2 diopters. As is clear from this formula, the shorter is the focal length f of an eyepiece, the greater is the range of dioptry variations available with a slight amount .DELTA.d of movement of the eyepiece. When the eyepiece is made movable for dioptry adjustment, a space is required for permitting movement in the optical device. From the above discussion, however, it is apparent that the dioptry adjustment by movement of the eyepiece is suitable for use in an optical device whose eyepiece has a relatively short focal length or an optical device which has sufficient room for movement of the eyepiece.
Within the viewfinder in a single-lens reflex camera, an image of an object formed by an objective lens and projected on a focusing plate is generally magnified by an eyepiece having a positive refracting power, so that the focal length of the eyepiece must be longer than the length of the optical path from the eyepiece to the focusing plate. However, a penta prism for reversing an image has to be provided between the focusing plate and the eyepiece in a conventional viewfinder of a single-lens reflex camera. The optical path length defined above is normally longer than 50 mm, in the case of a 35 mm single-lens reflex camera. Therefore, the focal length of the eyepiece is about 55 mm to 70 mm which is longer than the optical path length. On the other hand, in order to cover individual difference in diopter a sufficient latitude of approximately 2 diopter is needed. As is clear from the above analysis of the dioptry variations, an eyepiece having a focal length of 55 to 70 mm is required to be moved within the range of 6 to 10 mm in order to ensure a dioptry latitude of 2 diopters. However, in the case of a 35 mm single-lens reflex camera, the eyepiece must be provided in a limited space behind the penta prism, leaving no extra space for such a wide range of movement. In addition, the greater movement of the eyepiece, results in an undesirable, increasing rate of change in viewfinder magnification. For the above reasons, means for continuous dioptry adjustment has not been provided in the viewfinder of conventional 35 mm single-lens reflex cameras. The only available method for dioptry adjustment in the viewfinder of a 35 mm single-lens reflex camera is that of selectively mounting separate attachment lenses behind the eyepiece in accordance with the individual difference in dioptry. However, this method requires several kinds of attachment lenses suited to the individual dioptry differences to be provided for interchanging use. In addition, attachment lenses positioned behind the eyepiece section cause the field of view to be partially eclipsed or to become poorly visible.
The applicant of the present invention has proposed in Japanese Patent Application No. 53-34718 a dioptry adjustable eyepiece system characterized in that the eyepiece comprises a negative lens element movable along the optical axis and a stationary positive lens element located at the eye or rear side of the negative lens element and is designed to have a positive refractive power as a whole with the focal length of the eyepiece being comparatively long. In the same application, the refactive power .phi.1 of the movable negative lens is selected within the following range as a desirable embodiment mode; EQU -0,04.ltoreq..phi.1.ltoreq.-0.01
or the negative lens is a positive meniscus lens on the object side. Proposed also in the same application are dioptry adjustable eyepieces, wherein the lens elements are correlated as below; EQU 0&lt;1/r.sub.3 .ltoreq.0.045
and EQU 0.9.ltoreq.r4/r5.ltoreq.1.2
wherein r3 and r4 are the radii of curvature of the negative lens on the object side and the eye side, respectively, while r5 is the radius of curvature of the positive lens on the object side. Also proposed is a dioptry adjustable eyepiece having more than 50 mm of composite focal length and wherein the negative lens moves less than 5 mm for the desired adjustment.
The present invention is contemplated to provide an adjustment mechanism for such a dioptry adjustable eyepiece system to be used in a single-lens reflex camera.
In the conventional dioptry adjustment of an eyepiece, as the entire eyepiece lens system is moved back and forth along the optical lens, the lens system is held by a cylinder which is guided by a screw or a groove cam for axial movement by their relative rotation. With the present invention, however, only a lens element on the side of an object i.e., between another outer lens and a penta prism is moved. Therefore it is difficult to employ such a cylindrical construction. If the cylindrical construction is to be employed for the optical system of the present invention, at least two cylinders are required for the fixed lens and movable lens, and still another may be needed for the operation of the lens. In the case of such two cylinder structure, as the movable lens element is located interior to the stationary lens element, the eyepiece section must project considerably from the camera body, e.g. from the top cover due to the fact that the cylinder for the movable lens element should extend to the back surface of the camera to be accessible from the outside for adjustment with room for its movement being left, and that the cylinder for the fixed lens element should extend considerably rearwardly.
Furthermore, the central portion of the top of a single-lens reflex camera is shaped to follow the shape of the roof surface of the penta prism, and as a result, the eyepiece lenses are often formed nearly rectangular. With the construction wherein lenses are supported in cylinders for rotation, the lenses must be made circular, and are unsuitable for a single-lens reflex camera. In addition, the eyepiece is arranged in an extremely narrow space between the penta prism and the rear wall of the top cover, wherein it is difficult to provide a complex mechanism for lens movement.